Many moons ago, the “next” version of Zen Cart was given the name 1.6.0. As time moved on and features were added to the code for this release, it became clear that it was going to be different enough from the 1.5.x branch of Zen Cart that a bump in the major number made sense. So the release name was changed to 2.0. But many other things still used the old name – which was kind of confusing.
As of today though, the renaming is done.